The cellular architecture that is widely used by conventional mobile communication networks may include four basic elements for end-to-end, E2E, data transport and managed communication: the user equipment, UE, the radio access network, RAN, the core network, CN, and the operation, administration and maintenance, OAM, system. Currently deployed global system for mobile communication, GSM, wideband code-division multiple access, WCDMA, and universal mobile telecommunications system, UMTS, the so-called the 2nd generation, 2G, and the 3rd generation, 3G, mobile communication networks, conventionally use RANs comprising two geographically separated sites: radio base stations, RBS; and RBS controllers, for example a base station controller BSC and a radio network controller, RNC. After launching the 4th generation, 4G, long term evolution, LTE, mobile network, two separated sites belonging the 2G and 3G legacy RAN have been merged into a single site, the eNode B, eNB, comprising 2 sub-sites: a sub-site with baseband units, BBU, and a sub-site with the radio units, RU.
Various types of topologies for the field deployment of fronthaul optical network over legacy 2G/3G/4G networks have been recommended in the common public radio interface, CPRI, specification. One popular topology is the main-remote topology which has a star-network layout in which a BBU is remotely connected to a number of remote radio units, RRU, with a typical link distance of between a few hundred meters and 2 km. The link paths between the BBU and the RRUs are known as CPRI links. Conventionally, point-to-point, P2P, of single mode fibre, SMF, based duplex or simplex connections have been used by CPRI links. The frequency bands used by RU/RRU are usually below 3 GHz, which may enable implementation of flexible bandwidths from a few hundred KHz up to 20 MHz. With the use of arrayed antennas, for example a 4×4 antenna array, a data transmission rate of up to of 300 Mbit/s can be achieved between RRUs and UEs. To be able to support multiple RRUs with a flexible combination of different bandwidths for field deployment of RBSs, a system bandwidth of 100 MHz-200 MHz is commonly used to design legacy BBUs.
With the rapid growth of mobile communications in recent years, mobile communication systems are now required to support much larger system capacities with higher data rates over large coverage areas in a high-mobility environment. To satisfy such demands, a 5th generation mobile network, 5G, has been recently proposed. One of the basic requirements for the 5G network being outlined by the standardization bodies is that the 5G network shall deliver various types of services to UEs with ultrahigh peak data rates, e.g. tens of Gbit/s peak data rates for both uplink and downlink transmissions.
In order to achieve the desired ultrahigh peak data rates, a high-degree arrayed antenna system, for example 8×8 or 16×16 antenna arrays, and a 5G-Radio with ultra-wide system bandwidth, e.g. over 1 GHz, may be used. This is because the peak transmission data rate increases with increasing the system bandwidth as well as the number of arrayed antennas. To be able to design a compact 5G-Radio, it is desired to directly integrate arrayed antenna system into the 5G-Radio. Since the size of the antenna element decreases with increasing operating frequency, the 5G-Radio may be designed to be operated at the high frequency bands, for example 28 GHz.
Alternatively, the CPRI line bit rate of 10 Gbit/s with system bandwidth of 100-200 MHz conventionally designed for 2G/3G/4G-enabled legacy BBUs may be adapted for 5G-enabled BBUs, particularly during early stage field development of 5G-Radios that need to be integrated into the legacy 2G/3G/4G radio networks. Therefore, in order to satisfy the ultra-wide bandwidth of 5G-Radio, a number of BBUs designed for the legacy LTE, 4G, network may be used to provide bandwidth aggregation to implement the 5G-Radio. Taking 800 MHz bandwidth 5G-Radio as an example, one may make use of eight 100 MHz LTE-BBUs or four 200 MHz LTE-BBUs to implement bandwidth aggregation in order to provide 800 MHz bandwidth for 5G-radio.
One of challenges for the field deployment of the 5G-Radio network is the mismatch in optical distribution network, ODN, topology between the legacy 2G/3G/4G and the new 5G networks. In contrast to a conventional BBU-centralized star topology used by CPRI transport within 2G/3G/4G sites, the 5G-Radio now becomes the centralized point in the star topology where a single 5G-Radio has to be connected to a number of BBUs for CPRI transport. One of major problems to deploy a 5G-Radio network over and/or on top of legacy 2G/3G/4G networks is the significant increase of number of fibres, which could be up to a factor of 10 or more. For example, consider an 8 transmitter/receiver duplex-SMF interfaced transceiver is designed for 5G-Radio, and for the sake of bandwidth aggregation, a 5G-Radio centralized star topology is used to cross-connect a 5G-Radio with a cluster of four BBUs. With the most simple site configuration of 3-sectors and a single branch, 48 SMFs will be needed in order to support data steam transport over CPRI links for three 5G-Radios, which is a factor of 8 increase in the number of fibres compared to a similar 3-sector site with 6 SMFs in 2G/3G/4G cases. Such a drastic increase in the of number fibres for a single site is not acceptable by mobile operators due to the extreme high cost of the fibre roll-out and/or the cost of leased fibres in the existing 2G/3G/4G radio networks.
One of well-known methods to reduce the number of fibres is to make the use of dense wavelength division multiplexing, DWDM, technologies. Using DWDM, it is possible to reduce a large number of SMFs down to a single SMF. DWDM technologies also enable the deployment of cascaded-chain and/or ring network topologies for fronthaul optical networks, with transport link protection. Unfortunately, the commercial available off-the-shelf key components used by DWDM technologies, for example, transponders, arrayed waveguide gratings, AWG, wavelength selective switches, WSS, erbium-doped fibre amplifiers, EDFA, etc., are very expensive. This is because these components are usually designed to satisfy highly demanding requirements in terms of providing high link budget, high thermal stability and high flexibility for channel-plans with the possibility of specific band-bypass/band-filtering etc. and are designed for long-haul transport networks.